Liquid crystal display using a plurality of light adjusting sheets angled at 5 degrees or more

ABSTRACT

A light adjusting sheet for a planar lighting device includes a plurality of convex and concave streaks arranged alternately and approximately parallel to each other on one surface. The streaks may have a cross section of a sine curve. Another light adjusting sheet consists of a plurality of single sheets each having the above configulation, stacked together. A planar lighting device includes the light adjusting sheet of the above configulation, wherein peak lines of the convex streaks and bottom lines of the concave streaks are arranged to make a predetermined angle with an axis of a linear light source. The liquid crystal display includes a liquid crystal display element placed on front of the planar lighting device of the above configulation.

FIELD OF THE INVENTION

This invention relates to a light adjusting sheet for a planar lightingdevice used for a liquid crystal display of personal computers or wordprocessors or a liquid crystal television set, and a planer lightingdevice and a liquid crystal display using the sheet, and in particularto a light focusing or adjusting sheet for a planar lighting device ofthe edge-light type suitable for a back light of a thin display and aplanar lighting device and a liquid crystal display using the sheet.

BACKGROUND OF THE INVENTION

Smaller, lighter and thinner word processors and personal computers havebeen developed every year, and these having the so-called laptop type ornotebook type size have become today's main trend. Since the liquidcrystal display of the laptop type or the notebook type is usually notluminescent by itself, it has a back light on its back surface toimprove visibility. The back light is required to be thin and toilluminate the display surface uniformly.

Usually, a planar lighting device is used as the back light, and manykinds of planar lighting devices are used. For example, as shown in FIG.1, one of them has a light source 11 on the back side of a diffusingplate 10, wherein the light is emitted from the back side of thediffusing plate 10 and emanates uniformly from the surface of thediffusing plate 10 or from a regulator plate 12 attached to the surfaceof the diffusing plate 10, as described in Japanese Patent ProvisionalPublication No. 257188/90.

However, the most widely used planar lighting device is that of aso-called edge-light type. The edge-light type planar lighting devicehas a light source on a side of a light guiding plate, wherein the lightemitted from the light source emanates uniformly from the surface of thelight guiding plate or from a plate attached to the light guiding plate,and is widely used because it is thin and light, etc.

Recently, along with the efforts to make thinner displays and colordisplays, there is an increasing demand for improved luminance of aplanar lighting device as the back light for displays. Since a user ofthe display looks at the display from the front side in most of thetime, it is necessary to regulate the direction of the emanated lightfrom the planar lighting device so that the light emanates efficientlyin the forward direction to improve the luminance using the same lightsource. However, if the light emanates within a too narrow range, aslight change in the direction of the user's sight will make the displayinvisible. Therefore, it is required to diffuse the light across anappropriate range.

The details are described here using this planar lighting device of theedge-light type as an example. In general, if a light source is providedon the edge side of a thin light guiding plate, a light which enters thelight guiding plate through the edge surface repeatedly experiencestotal reflections and almost no light leaves the light guiding platebecause of the difference in the refractive index between the air andthe light guiding plate. In the planar lighting device, an irregularreflection layer is provided on the back surface of the light guidingplate and a reflector plate is further provided on the back side of thelight guiding plate, so that the light which reaches the back side ofthe light guiding plate is irregularly reflected by the irregularreflection layer, and then goes out of the light guiding plate directlyor after being reflected by the reflector plate. The irregularreflection layer is often formed by the dot-printing using paintincluding beads of small diameters.

However, the light emitted from the light source on the edge surface ofthe light guiding plate usually goes out from the light guiding plate ata very small angle from the surface. That is, the light has a strongdirectionality, as shown in FIG. 2.

A more detailed explanation of the above problem is given here byreferring to FIG. 2. A transparent light guiding plate 20 has anirregular reflection layer (not shown) on its back surface. A reflectorplate 21 is provided on the back side of the light guiding plate 20, anda linear light source 22 is provided on the edge side of the lightguiding plate 20. The light emitted from the light source 22 travelsthrough the light guiding plate 20 and irregularly reflects at theirregular reflection layer provided on the back surface of the lightguiding plate 20. Then, the light either directly goes out of the lightguiding plate 20 or reflects at the reflector plate 21 and then goes outof the light guiding plate 20. The light from point A has a strongdirectionality making a very small angle with the surface of the lightguiding plate 20, indicated by B in FIG. 2.

Since it is rare that the user looks at the screen of the display atsuch a small angle with the surface of the light guiding plate 20, it isnecessary to change the angle of the emanating light. In order toimprove the directionality of the light, a method in which a lightdiffusing plate 23 is provided on the front surface of the light guidingplate 20 has been proposed, as shown in FIG. 3. The light diffusingplate 23 is a transparent plastic sheet applied with a paint containinga white pigment on its surface, or a transparent plastic sheet providedwith fine unevenness on its surface by the mat treatment, the crimptreatment, or such. By providing the light diffusing plate 23, the lightin the direction perpendicular to the diffusing plate 23 increases,indicated by C in FIG. 3.

FIG. 4 shows a detail of the light crystal display, illustrating thebasic structure of the planar lighting device of the edge-light type.The planar lighting device 30 mainly includes a light guiding plate 32consisting of a transparent plate having a dot pattern 31 on the backsurface, at least one linear light source 33 of a cathode ray tube(fluorescent lamp) provided on at least one side of the light guidingplate 32, a reflector plate 34 provided behind the light guiding plate32, and a light diffusing sheet 35 consisting of a resin plate includinga light diffusing material or a resin plate provided with crimps on itssurface, and a liquid crystal display element 36 is further provided infront of the planar lighting device 30 (refer to Japanese PatentProvisional Publication No. 244490/89, U.S. Pat. Nos. 4,775,222 and4,729,068). The dot pattern 31 is a light scattering printed dot patternformed on the back surface of the light guiding plate 32 so that theincident light from the light source 33 on the side goes out uniformlyfrom all parts of the screen of the display, and it may be called apseudo light source. The light diffusing sheet 35 functions in such away that the back light source, i.e. the dot pattern 31 as the pseudolight source, is not visible and the screen is seen as a uniformlyluminescent plane, when using the liquid crystal display screen.

However, in the device using the light diffusing plate 35, much of thelight is emitted in directions not needed for the user and the light inthe forward direction is scarce, so that the luminance of the light inthe foward direction from which the user sees is low.

Conventionally, in order to make the dot pattern 31 invisible andincrease the light diffusion efficiency. The following measures havebeen taken: i) coating a light diffusing material onto a plastic sheetsurface or introducing it into the inside of the sheet, and ii) creatinga crimp-like unevenness or a regular unevenness on the plastic sheet.

In the former measure i), the light diffusing sheet 35 consists of asheet formed by an extruder from a material made of a resin such aspolyester, polycarbonate or polymethylmethacrylate, mixed with a lightdiffusing material such as fine-powder of calcium carbonate, titaniumoxide, short glass fiber or silicone resin particles containingpolysiloxane bonds (refer to Japanese Patent Provisional Publication No.140343/78).

In the latter measure ii), the sheet having a crimp-like unevenness or aregular unevenness on its surface is provided on the front side of thelight guiding plate 32 or on at least one surface of the light diffusingsheet 35 (refer to Japanese Patent Provisional Publication No.257188/90). Such a sheet is called "a light adjusting sheet".

In one example of the latter measure ii), a light adjusting sheet 37consisting of a transparent prism sheet with convex streaks and concavestreaks each having a triangular cross section, as shown in FIG. 5, isprovided on the front side of the light guiding plate 32 so that theopposite surface with no convex streaks and concave streaks of the lightadjusting sheet 37 is in contact with the light guiding plate 32 in aliquid crystal display, as shown in FIG. 6. In such a planar lightingdevice of the liquid crystal display, the strongly directional light isredirected in the direction of the user's sight by the slopes of thetriangular cross section of the convex streaks and concave streaks, andthe emitting direction of the light is confined to a predetermined rangeand converged in the direction of the user of a wordprocessor or othersuch device, i.e. the normal direction of the screen, and emanates fromthe front surface of the light adjusting sheet 37, thus obtaining ahigher luminance in the screen.

However, in the former case i), of the planar lighting device with thelight diffusing sheet, if the amount of the light diffusing materialcontained in the light diffusing sheet 35 is increased to improve thelight diffusion, the amount of the emanated light is decreased becauseof the shielding characterisity of the light diffusing material,resulting in insufficient brightness in the screen, indicated by thedotted line D in FIG. 7.

On the other hand, in the latter case ii), of the planar lighting devicewith the light adjusting sheet, it utilizes the prism effect of theconvex streaks and concave streaks on the surface of the light adjustingsheet, and is superior to the former device in terms of the amount ofthe emanated light. However, since the directionality of the light istoo strong and the incident direction and the outgoing direction of thelight are too strictly defined, so that the light emanating direction isdeflected in the forward direction within a narrow angle range and thesight angle of the screen tends to be exceedingly narrow, as indicatedby E in FIG. 8.

Further, when the light adjusting sheet consisting of a prism sheet of aserrate cross section is placed in such a way that its flat back surfaceis brought into direct contact with the light guiding plate,interference patterns of Newton ring appear due to slight gaps generatedbetween the back surface of the light adjusting sheet and the lightguiding plate. The closer the convex streaks and the concave streaks areprovided to the final outgoing surface, the lattice-like partition linesof the liquid crystal display surface and the ridge lines and bottomlines of the prism sheet cause interference (the Moire phenomenon),depending on the pitches of the convex streaks and concave streaks.

Further, the inventor experimented with this planar lighting device andfound that there is a problem in that the irregular reflection layerprovided on the back surface of the light guiding plate is visible,although it can indeed alter the direction of the light efficiently sothat the light emanates in the direction of the user's sight. When adisplay using the liquid crystal is placed on the planar lighting devicewith the visible irregular reflection layer, the screen of the displayis very hard to see.

Therefore, a light diffusing sheet 35 is required in the planar lightingdevice when it uses the conventional light adjusting sheet.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a lightadjusting sheet for a planar lighting device in which the emanated lightdistribution can be concentrated in the forward direction of the screen,and no Moire fringes occur.

It is another object of this invention to provide a light adjustingsheet for a planar lighting device which gives greater luminance in theforward direction compared with the case when no light adjusting sheetis used, gives a wider sight angle than the conventional light adjustingsheet, and assures prevention of an optically tight contact with thelight guiding plate.

It is a further object of this invention to provide a light adjustingsheet for a planar lighting device which gives a suitable span in thedirection of the user's sight and an improved brighter screen.

It is a still further object of this invention to provide a lightadjusting sheet for a planar lighting device with reduced reflection atthe front surface thereof, with no interference fringes, and also withthe irregular reflection layer made invisible.

It is yet another object of this invention to provide a planar lightingdevice of an edge-light type which can illumimate a screen of a displaybrightly and uniformly.

It is yet further object of this invention to provide a liquid crystaldisplay in which the screen is bright and easily seen.

According to an aspect of the invention, a light adjusting sheet has onesurface provided with a plurality of convex streaks each having a crosssection of a convex are and a plurality of concave streaks each having across section of a concave arc alternately arranged approximatelyparallel to each other, and the other surface which is an opticallynon-smooth surface. Throughout the specification, the term "sheet"includes not only a sheet in the strict sense limited by the thicknessbut also thinner one which is usually called "film". Also, the term"approximately parallel" means that a peak line of each of the pluralityof the convex streaks and a bottom line of each of the plurality of theconcave streaks are approximately parallel to each other.

As described above, the light adjusting sheet according to the inventionhas one surface provided with a plurality of convex streaks each havinga cross section of a convex arc and a plurality of concave streaks eachhaving a cross section of a concave are alternately arrangedapproximately parallel to each other and the other surface of anoptically non-smooth surface, so that the planar lighting device usingthis light adjusting sheet has advantages in that the luminance in theforward direction is improved compared with one without the lightadjusting sheet, the dot pattern on the back surface of the lightguiding plate is not visible in the least, the light diffusion isadequate, the sight angle range is kept equivalent to that of a liquidcrystal display element itself, and the interference (Moire phenomenon)between the lattice-like partition lines of the liquid crystal displaysurface and peak lines of the convex streaks and bottom lines of theconcave streaks of the sheet is prevented.

Therefore, if this light adjusting sheet for a planar lighting device isused in a planar lighting device of the edge-light type or a planarlighting device which has a light source on the back side of a lightdiffusing plate, then a bright and very easy-to-see screen can beobtained by installing the planar lighting device in a liquid crystaldisplay. The light adjusting sheet for a planar lighting device in theinvention can be effectively used not only in a liquid crystal displaydevice but also in stores, houses, offices, etc. as a thin planar lightsource for thin facility lighting equipments.

The measurement procedure of the outgoing light luminance of the planarlighting device is described below. In FIG. 9, let us assume that thenormal angular coordinate of the light emanating surface of the planarlighting device 30 is 0°, and then the luminance of point P on theplanar lighting device 30 is measured at several positions within therange from -90° to +90° with the line which passes the point P and isparallel to the axis of the light source 33 as the axis, using aluminance meter 40. The maximum luminance value of these measurements isrepresented as 100%, and values at other positions are represented aspercentages. The relationship between the measurement positions and theluminance is plotted on a graph.

The solid line F and the dotted line G in FIG. 10 show the luminancedistributions of the planar lighting device of the edge-light type withthe basic structure shown in FIG. 4. The solid line F in FIG. 10 showsthe luminance distribution with a wider sight angle, and the dotted lineG in FIG. 10 shows the luminance distribution which has a greater amountof the outgoing light in the direction of the user (the normal directionof the screen).

In this lighting device, it is shown that the amount of emanated lightis small in the front direction while the user is at the position justin front of the lighting device (0°). In order to increase the luminancedistribution in the forward direction, there have been proposals inwhich the light adjusting sheet 37 with the prisms shown in FIG. 5 inwhich convex streaks and concave streaks with a serrate-shaped crosssection are formed on one or both sides of the sheet is used, or inwhich fine unevenness is directly formed on the light emanating side ofthe light guiding plate or on the reflector plate side. These convexstreaks and concave streaks provided on the light adjusting sheet or onthe light guiding plate are formed parallel to the light source axis.However, in this case, the angle for the maximum amount of emanatedlight differs depending on models of the planar lighting device.Therefore, when a light adjusting sheet which has prisms with a specificconfiguration is used, a certain model of the lighting device producesgood emanated light characteristics, while another model of the lightingdevice produces not as good results.

The plurality of the convex streaks and the concave streaks of the lightadjusting sheet function to concentrate the outgoing light from thesheet more in the normal direction of the screen. As shown in FIG. 11,these plural convex and concave streaks are provided on the lightemanating surface of the light adjusting sheet, and each peak 51a of theconvex streaks 51 has a cross section of a convex arc and each bottom52a of the concave streaks 52 has a cross section of a concave arc. Theangle α between the two slopes of each convex streak (this angle isdefined as an angle made by two tangent lines in the middle of the twoslopes in this invention) is in the range of 30° to 150°, and preferablyof about 60° to 120°. It is difficult to form the convex and concavestreaks if the angle is less than 30°, and the light concentrationcapability of the sheet may decrease if the angle is greater than 150°.The radius β of curvature of these convex arcs and concave arcs in crosssections of the convex and concave streaks is determined by the depth γof the convex and concave streaks, the angle α made by the slopes, thepitch δ, etc., and it is preferably 10 to 100 μm. The convex and concavestreaks may have a lateral cross section of a sine curve represented bythe equation Y=α. sin bX (Y: Coordinate in the depth-wise direction inmm units, X: Coordinate in the direction perpendicular to the streaks inmm units, "a" and "b": factors). Though the parameters may be variable,the preferable range of "a" may be 200 to 20000 and that of "b" may be0.005 to 1 according to experiments conducted by the inventors. If theconvex and concave streaks have a cross section of a sine curve asdescribed above, the light is converged by the streaks into the forwarddirection, resulting a very easy-to-see light when emanated.

As shown above, the light adjusting sheet in this invention has theplurality of the convex streaks each having a peak of a cross section ofa convex arc and the concave streaks each having a bottom of a crosssection of a concave arc, so that the sheet has functions of adequatelight concentration, light diffusion and light directionality of theemanating light to forward direction. Therefore, the surface providedwith the convex and concave streaks in the light adjusting sheet mayinclude flat surfaces which form less than 50% of the total area of thecurved surface between each of the convex and concave streaks. In thiscase, there is a tendency that the light adjusting sheet has anincreased light concentration function.

The thickness of the light adjusting sheet is 50 μm or greater,preferably 90 to 300 μm.

The material of which the light adjusting sheet consists is not limited,as long as it is a transparent organic or inorganic material, that is atransparent material such as glass or transparent synthetic resin formedinto a sheet. A synthetic resin sheet is particularly preferable. Forthe transparent synthetic resin, polycarbonate, polymethyl methacrylate,polyester, cellulose synthetic resins, polystyrene, polyvinylchrolideand such are preferable. The light adjusting sheet is preferably made ofthe same type of resin as the base material of the light diffusing sheetwhich will be described later.

Preferable methods for fabricating the light adjusting sheet made of thesynthetic resin include a method in which a transparent synthetic resinis extruded and goes through an embossing roll to have convex andconcave streaks formed on the surface, a method in which a syntheticresin is pressed using a die plate with convex streaks and concavestreaks provided on the surface, a method in which a synthetic resin isinjected into a die with convex streaks and concave streaks provided onthe inner surface, etc.

The back surface, which does not have convex streaks or concave streaks,may be a non-smooth surface. For the non-smooth surface, those on whichfine and random unevenness, for example, may be formed. The size anddepth of the unevenness is not limited as long as there is no opticallytight contact of a size recognizable by the naked eye when this surfaceis brought in contact with a smooth surface. The non-smooth surface canalso be a moderate wave formed surface. For the method of forming thenon-smooth surface, the method in which a pattern on a roll or a die istransferred to the surface at the same time of forming the surface, andthe method in which calendering, sand blasting, chemical etching, themat treatment, pressing, etc. are used to give unevenness to an alreadyformed sheet, are applicable. The particularly preferable examples ofthe methods of forming the non-smooth surface are the mat treatment, thesand blasting method, the press method and such.

For example, in the case of the planar lighting device without a lightguiding plate as shown in FIG. 1, the back surface of the lightadjusting sheet is not to be in contact with anything, so that the lightadjusting sheet with a flat back surface can be suitably employed. Inthe case of the planar lighting device with a light guiding plate asshown in FIG. 4, the light adjusting sheet is used with the lightguiding plate in contact with its back surface, and interference fringesof Newton ring may result because of the slight gap between this backsurface of the light adjusting sheet and the light guiding plate if theback surface of the light adjusting sheet is a flat surface. Therefore,in such cases, it is preferable to provide unevenness on the backsurface of the light adjusting sheet.

For example, it is preferable to form fine unevenness on the backsurface formed by sand blasting and such as shown in FIG. 11, becausethis will eliminate interference fringes of Newton ring and also thelight will be diffused. In addition, the directionality of the lightdecreases if the back surface of the sheet is provided with convex andconcave streaks of a cross section of a continuous curve shown in FIG.20 or of a triangular cross section shown in FIG. 21. According to theexperiments by the inventors, even better results can be obtained bymaking the period of the uneven streaks on the back surface of the lightadjusting sheet the same as the period of the convex and concave streaksof the front surface and making the amplitude less than a half of thatof the front surface.

As described above, the back surface of the light adjusting sheetopposite to the surface provided with the convex streaks and concavestreaks can be changed in various ways to produce different results.Therefore, a suitable combination with the convex and concave streaks ofthe front surface should be chosen, depending on the refractive indexand the thickness of the light guiding plate, the refractive index ofthe light adjusting sheet, etc.

Also, the light adjusting sheet may contain a light diffusing material.The light diffusing material gives an ability to diffuse light to thelight adjusting sheet. For the light diffusing material, white pigmentssuch as calcium carbonate powder, titanium oxide powder and zinc white,white inorganic powder such as alumina powder, silica powder and whiteclay, glass beads, glass fiber, synthetic resin powder with a refractiveindex different from that of the light adjusting sheet, etc., arepreferable.

Also, regarding the diffusion material content, there is little effectif the light diffusion material is equal to or less than 0.01 weightunits against 100 weight units of the transparent material of glass,synthetic resin or such, of which the light adjusting sheet consists,and then the effect does not increase any more. On the other hand, thelight transmission performance rather deteriorates if it is 10 weightunits or more. Therefore, the range of 0.01 to 10 weight units ispreferable, and 0.1 to 5 weight units is more preferable.

There are several methods for making the light adjusting sheet containthe light diffusing material, that is, i) a method in which the lightadjusting sheet is formed by a transparent material which is alreadymixed with the light diffusing material, ii) a method in which the lightadjusting sheet is formed by stacking a plurality of layers of atransparent material layer which mainly consists of the light adjustingsheet and a transparent material which includes a light diffusingmaterial, and iii) a method in which a coating material consisting of atransparent material which includes a light diffusing material is coatedon the surface of the light adjusting sheet, etc..

The surface of the sheet on which the convex and concave streaks areprovided is used as the front surface through which the light emanates.The light transmitted through the front surface is refracted by theconvex and concave streaks and converged in the normal direction, i.e.the front direction, of the light adjusting sheet. For example, when thesheet is used in a planar lighting device of the edge-light type, theopposite surface which is not provided with the convex and concavestreaks is in contact with the front surface of the light guiding plate.Thus, the light emanated from the front surface of the light guidingplate at a low angle goes through the light adjusting sheet can emanateas a converged light in the forward direction.

More detailed explanations are given below, by referring to an exampleof the light adjusting sheet of this invention installed in a planarlighting device of the edge-light type. In the planar lighting device ofthe edge-light type, a light emitted from a linear light source providedon the edge side of the light guiding plate enters the light guidingplate through the edge surface. The light which has entered the lightguiding plate is irregularly reflected by the irregular reflection layerand either directly goes out from the front surface or goes out of thefront surface after being reflected by the reflector plate provided onthe back surface of the light guiding plate. The light emanating fromthe light guiding plate is very directional, as shown in FIG. 2. Sincethe light adjusting sheet is provided on the front surface of the lightguiding plate, the light enters the light adjusting sheet.

Here, if the opposite surface of the light adjusting sheet which is tobe in contact with the light guiding plate has fine unevenness, theopposite surface is not in tight contact with the light guiding plate,and therefore interference fringes of the Newton ring which result fromfine gaps at a tight contact do not appear. Further, the light taking-inefficiency (a ratio of the light entered the light adjusting sheetwithout reflection to the light which goes out of the light guidingplate) is improved.

Then, the light travels through the light adjusting sheet emanates fromthe front surface having the convex and concave streaks. Here, the lightis converged in the forward direction by the convex and concave streaks,and the irregular reflection layer such as dot printing provided on theback surface of the light guiding plate becomes invisible, and thus thelight emanates as an easy-to-see light for the user. If a diffusingmaterial is mixed in the light adjusting sheet, the light is suitablydiffused.

When a liquid crystal display element is installed in front of the lightadjusting sheet, an optical contact of the light adjusting sheet and theliquid crystal display element may not occur by the uneven streaksprovided on the surface of the light adjusting sheet.

The light adjusting sheet configured as described thus far is used for aplanar lighting device. As shown in FIG. 12, the planar lighting device30 includes at least one light adjusting sheet 50 with the configurationdescribed above, a light guiding plate 32 provided behind the lightadjusting sheet 50 with a printed dot pattern 31 on the back surface, atleast one linear light source 33 provided on at least one side of thelight guiding plate 32, and a reflector plate 34 provided behind thelight guiding plate 32. As shown in FIG. 4, at least one light diffusingsheet 35 may be provided as necessary between the light adjusting sheet50 and the light guiding plate 32 or in front of the light adjustingsheet 50, i.e. the light emanating end. Then, a liquid crystal displayis obtained by installing a liquid crystal display element 36 in frontof the planar lighting device 30.

In the planar lighting device, each of the convex and concave streaks ofthe light adjusting sheet 50 makes a predetermined angle with an axis ofthe light source 33. In order to provide the convex and concave streaksto make a predetermined angle with the axis of the light source 33, asquare sheet provided with a plurality of convex streaks and concavestreaks which are parallel to each other and which make a predeterminedangle with an edge line of the sheet may be used as a light adjustingsheet, wherein the sheet is arranged so that the edge line of the sheetis parallel to the axis of the light source 33.

As shown in FIG. 13, the plurality of the convex and concave streaks areformed on one side of the sheet in such a way that each peak line 54 ofthe streaks makes an angle θ with the edge 55 of the sheet. The angle θis preferably in the range of 5°≦θ≦85°. If θ exceeds 85°, it may not bepossible to obtain a sufficient light concentration effect. If θ issmaller than 5° or θ is 85° or greater, the peak lines of the streaks onthe light adjusting sheet and the pitch lines between dots of the liquidcrystal display may cause Moire fringes.

Additionally, Moire fringes may appear between specific patternsdisplayed on the liquid crystal display element and the streaks of thelight adjusting sheet, though the angle θ is in the range of 5°≦θ≦85°.In order to effectively prevent this problem, the angle θ is preferablyapproximately 36°, determining by frequency of occurence of the patternsdisplayed on the liquid crystal display element. If it is not requiredto consider occurence of Moire fringes with the patterns displayed onthe liquid crystal display element, the angle θ is set by determiningthe luminance distribution as a function of the measurement directionwith the procedure described above, and determining the value at whichthe emanated light distribution is concentrated towards the front of thescreen.

According to another aspect of the invention, a light adjusting sheetfor a planar lighting device includes a plurality of sheets, each sheetbeing provided with a plurality of convex streaks and concave streaksparallel to each other, stacked together in such a way that peak linesof the streaks make an angle of 5° or greater.

The representative example of the light adjusting sheet consists of twosquare sheets, each square sheet being provided with a plurality ofconvex streaks and concave streaks approximately parallel to each other,stacked together in such a way that peak lines of the streaks of onesquare sheet and the other make an angle 5° or greater, the peak linesof the streaks of one square sheet make an angle of 5° to 85° in theclockwise direction with the sheet edge, and the peak lines of thestreaks of the other square sheet make an angle of 5° to 85° in thecounterclockwise direction with the sheet edges.

In the light adjusting sheet of the two sheet structure, as shown inFIG. 14, supposing the angle between the peak lines of the streaks ofone square sheet and the sheet edge be θ in the counterclockwisedirection, then the angle θ' in the clockwise direction for the othersquare sheet is preferably in the range of 5°≦θ'≦85°. Also, the angle θfor one square sheet and the angle θ' for the other square sheet arepreferably set in opposite directions, clockwise and counterclockwise,against the sheet edge, with the same absolute value. However, asdiscussed above, the angle between the peak lines of the streaks of onesquare sheet and the peak lines of the streaks of the other square sheetmust be 5° or greater, so that these square sheets must be stacked insuch a way that the absolute value of the difference between the anglesθ and θ' is not smaller than 5°. If |θ-θ'|<5°, then Moire fringes mayresult from the convex streaks and concave streaks of these squaresheets.

When three or more square sheets are stacked together, the sheetstacking is also conducted in such a way that the peak lines of thestreaks of one square sheet make an angle of 5° or greater with the peaklines of the streaks of any other square sheet.

Next, each element which composes the planar lighting device 30 will bedescribed.

First, the light adjusting sheet 37 (50, 60) has the configurationdescribed above. It is possible to use a plurality of the lightadjusting sheets stacked together, or to use it jointly with a lightdiffusing sheet 35 of the conventional type.

For the linear light source 33, a cathode ray tube is usually used. Thecathode ray tube may be either a cold cathode ray tube or a hot cathoderay tube. There is no limitation on the size and such of the cathode raytube. The light source(s) is provided on both left and right sides or onone side of the light guiding plate 32. Also, the light sources may beprovided on three or all four sides of the light guiding plate 32.

The light guiding plate 32 is made of a material with good transparencysuch as glass, polycarbonate, polyester and polymethylmethacrylate, anda printed dot pattern 31 is applied on its back surface, i.e. thesurface on the reflector plate side, to cause irregular reflection insuch a way that the light from the light source 33 emanates uniformlyfrom each position of the light emanating surface. The thickness andsuch of the light guiding plate 32 are not limited.

For the reflector plate 34, any plate can be used without limitation aslong as it functions to block and reflect a light, such as a resin platewith a white pigment blended into it, a foamed resin plate, a resinplate with a metal vapor coating or a metal plate. The reflector plate34 is placed on the non-emanating side of the light guiding plate 32,and is in contact with the printed dot pattern 31 of the light guidingplate 32.

The light diffusing sheet 35 diffuses the light so that the user doesnot see the configuration of the printed dot pattern 31 on the lightguiding plate 32. There are several types, that are, a type in which alight diffusing material is mixed, a type in which a light diffusingmaterial is coated, a type in which a random unevenness is formed, and acombination type of the above types. The thickness of this sheet is notlimited but is usually 10 μm or greater, and preferably 20 to 300 μm. Ifthe thickness is less than 10 μm, sufficient diffusion characteristicsmay not be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be explained in more detail in conjunction withappended drawings, wherein:

FIG. 1 is an explanatory view showing the conventional planar lightingdevice.

FIG. 2 is an explanatory view showing the light distributioncharacteristics of the conventional planar lighting device of theedge-light type.

FIG. 3 is an explanatory view showing the light distributioncharacteristics of the conventional planar lighting device of theedge-light type.

FIG. 4 is a cross-sectional view showing the conventional liquid crystaldisplay device of the edge-light type.

FIG. 5 is a perspective view showing the conventional light adjustingsheet for a planar lighting device.

FIG. 6 is a cross-sectional view showing the conventional liquid crystaldisplay device of the edge-light type.

FIG. 7 is a luminance distribution diagram for the planar lightingdevice of the edge-light type.

FIG. 8 is a luminance distribution diagram for the planar lightingdevice of the edge-light type.

FIG. 9 is a schematic diagram showing the luminance measurementprocedure for the planar lighting device.

FIG. 10 is a luminance distribution diagram for the planar lightingdevice of the edge-light type.

FIG. 11 is a perspective view showing a light adjusting sheet accordingto the invention.

FIG. 12 is a cross-sectional view showing a planar lighting device ofthe edge-light type according to the invention.

FIG. 13 is a plan view showing a planar lighting device of theedge-light type according to the invention.

FIG. 14 is a plan view showing a planar lighting device of theedge-light type according to the invention.

FIG. 15 is a cross-sectional view showing a light adjusting sheetaccording to the invention.

FIG. 16 is a cross-sectional view showing a light adjusting sheetaccording to the invention.

FIG. 17 is a cross-sectional view showing a light adjusting sheetaccording to the invention.

FIG. 18 is a cross-sectional view showing a planar lighting device ofthe edge-light type using the light adjusting sheet shown in FIG. 17.

FIG. 19 is an explanatory view showing the light distributioncharacteristics of the planar lighting device of the edge-light typeaccording to the invention.

FIG. 20 is a cross-sectional view showing a light adjusting sheetaccording to the invention.

FIG. 21 is a cross-sectional view showing a light adjusting sheetaccording to the invention.

FIG. 22 is a cross-sectional view showing a light adjusting sheetaccording to the invention.

FIG. 23 is a persepective view showing a light adjusting sheet accordingto the invention.

FIG. 24 is a cross-sectional view showing a conventional sheet which canuse with a light adjusting sheet according to the invention.

FIG. 25 is a cross-sectional view showing a sheet according to theinvention.

FIG. 26 is a plan view showing a liquid crystal display device of theedge-light type according to the invention.

FIG. 27 is a plan view showing a liquid crystal display device of theedge-light type according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiments according to the invention will be described.

Embodiment 1

As shown in FIG. 15, a plurality of convex streaks 51 and concavestreaks 52 are alternately formed parallel to each other on the lightemanating surface of a square sheet made of polycarbonate with a meltindex of 4.0 (290° C. 1.9 kg). A peak 51a of each convex streak 51 has alateral cross section of a convex arc, and a bottom 52a of each concavestreak 52 has a lateral cross section of a concave arc. An angle betweenthe two slopes of each convex streak 52 (an angle between two tangentlines at the middle of each slope) is roughly a right angle. The totalthickness of the sheet is 200 μm, the height from the bottom 52a of theconcave streaks 52 to the peak 51a of the convex streaks 51 is 120 μm,the pitch of the convex streaks 51 and the pitch of the concave streaks52 are both 350 μm. The radius of curvature for both the peak 51a ofeach convex streak 51 and the bottom 52a of each concave streak 52 is 67μm. The convex streaks 51 and the concave streaks 52 are formed by thethermal press method. The back surface 53 of the sheet is provided withfine unevenness by the mat roll method. Such is the configuration of thelight adjusting sheet 50 which has a plurality of the convex streaks 51and the concave streaks 52 on the front surface and the unevenness onthe back surface 53 (a light adjusting sheet (a)).

Performance Test

The forward luminance and the luminance ratio are measured and theappearance is observed for the planar lighting device equipped with thelight adjusting sheet described above.

I) Luminance

The cathode ray tube (light source) 32, the light guiding plate 32, thereflector plate 34, and the light diffusing sheet 35, described below,are used to compose the planar lighting device for the measurements ofthe screen brightness.

Cathode ray tube: Cold cathode ray tubes of 3.0 mm diameter and 130 mmlength.

Light guiding plate: 130 mm long, 260 mm wide, 3.0 mm thick. Thematerial is an acrylic resin. Dot printing for irregular reflection isapplied on the back surface.

Reflector plate: 100 μm thick. The material is polycarbonate mixed with20 wt % white pigment (titanium oxide).

Light diffusing sheet: The material is polycarbonate mixed with 10 wt %calciumcarbonate.

In FIG. 12, which shows the basic structure of the planar lightingdevice of the edge-light type, the cathode ray tube 33 as a light sourceis placed on each side of the light guiding plate 32 which has theprinted dot pattern 31 on its back surface. The reflector plate 34 isplaced behind the light guiding plate 32, and the light adjusting sheet50 is placed in front of the light guiding plate 32. The light adjustingsheet 50 is placed in such a way that the surface with the convexstreaks 51 and the concave streaks 52 is to be the light emanatingsurface and that the convex streaks 51 and the concave streaks 52 areparallel with the axis of the cathode ray tubes 33.

With the planar lighting device 30 described above, the luminance ismeasured in the normal direction (0°) and in the direction of 30° fromthe normal direction. The procedure to measure the luminance is asdescribed before.

II) Appearance

For the planar lighting device 30 described above, the screen is checkedto see if there is any problem in appearance.

Embodiments 2 to 4

Light adjusting sheets (b), (c) and (d) are obtained by the sameprocedure as for the sheet (a), except that the sheet material, the sizeof the convex streaks and concave streaks and the forming method ofunevenness of the back surface are changed to the items listed inTable 1. For the planar lighting devices equipped with either lightadjusting sheet of (b), (c) or (d), a performance test is conducted inthe same manner as for Embodiment 1. The light adjusting sheet (d) ofEmbodiment 4, as shown in FIG. 16, has a back surface of non-smoothsurface 53 formed into a moderate wave form surface by the press method.

Control Example 1

A performance test is conducted in the same manner as for Embodiment 1for a planar lighting device which is not equipped with the lightadjusting sheet of this invention.

Control Examples 2 to 3

The light adjusting sheets (e) and (f) are obtained by the sameprocedure as for the sheet (a), except that the sheet material. The sizeof the convex streaks and the concave streaks and the forming method ofthe unevenness of the back surface are changed to the items listed inTable 1. A performance test is conducted in the same manner as forEmbodiment 1 for the planar lighting device equipped with each of thelight adjusting sheets (e) or (f). The light adjusting sheet (e)corresponds to the conventional sheet shown in FIG. 5.

The sheet material, the size of the convex streaks, the forming methodof the back surface, and the results of the performance tests for eachlight adjusting sheet of the Embodiments and the Control examples aresummarized in Table 1. In Table 1, the marks represent as following:

*: A light diffusing sheet which has a random uneven surface on thefront surface and a mat surface on the back surface.

◯: No problem

X: There is some problem

As clearly shown in Table 1, compared with those of the Controleexamples, the light adjusting sheets of the Embodiments have superiorluminance in the normal direction (0°) and in the direction of 30° fromthe normal direction, and they are superior in appearance as well.

                                      TABLE 1                                     __________________________________________________________________________             Embodiments              Control examples                                     1     2     3      4     1    2     3                                __________________________________________________________________________    Light adjusting                                                                        a     b     c      d     none e     f*                               sheet                                                                         Material poly- poly- polymethyl                                                                           poly- --   polymethyl                                                                          poly-                                     carbonate                                                                           carbonate                                                                           methacry-                                                                            carbonate  methacry-                                                                           carbonate                                             late              late                                   Total thichness                                                                        200   170   400    250   --   500   250                              (μm)                                                                       Concave depth                                                                          120   70    180    120   --   180   --                               (μm)                           --                                          Peak angle (°)                                                                  90    80    100    75    --   90    --                               Pitch (μm)                                                                          350   190   430    200   --   360   --                               Radius of curv-                                                                        67    50    30     30    --   0     --                               ature of convex                                                               and concave                                                                   streaks (μm)                                                               Forming method                                                                         Mat roll                                                                            Sand blast                                                                          Random un-                                                                           sine curve                                                                          --   Mirror                                                                              --                               of back surface      even die                                                                             with the   surface                                                     press  same phase                                                                    as light                                                                      emanating                                                                     surface,                                                                      press with                                                                    15 μm con-                                                                 cave                                              Luminance                                                                              405   400   415    420   150  530   330                              (0°, cd/m.sup.2)                                                       Luminance                                                                              380   385   375    370   250  45    360                              (0°, cd/m.sup.2)                                                       Appearance                                                                             ◯                                                                       ◯                                                                       ◯                                                                        ◯                                                                       X    X     ◯                                                      Dots are                                                                           Dots are                                                                 visible.                                                                           visible.                                                                      Bright po-                                                                    ints by a                                                                     tight con-                                                                    tact.                                  __________________________________________________________________________

Embodiment 5

FIG. 17 shows a part of a light adjusting sheet for a planar lightingdevice in Embodiment 5, and FIG. 18 shows a planar lighting device ofthe edge-light type in which the light adjusting sheet shown in FIG. 17is used.

In FIG. 17, the light adjusting sheet 50 has a front surface 56 providedwith convex streaks and concave streaks forming a sine curved surfaceand a back surface 53 of unevenness.

In FIG. 18, the numeral 32 represents a light guiding plate consistingof a polymethyltmethacrylic plate with a 3 mm thickness, and adot-printed irregular reflection layer (not shown) is provided on theback surface of the light guiding plate 32. The particle diameter of thebeads used in the dot printing is in the range of 0.5 to 1.5 mm thenumeral 34 represents a reflector plate. The reflector plate 34 isprovided on the back surface of the light guiding plate 32. Thereflector plate 34 reflects the light emanating from the back surface ofthe light guiding plate 32 back to the light guiding plate 32. Thenumeral 50 represents a light adjusting sheet consisting of apolycarbonate transparent sheet. The light adjusting sheet 50 is made bythe method in which polycarbonate is formed into a 200 μm thick sheetwith convex streaks and concave streaks having an amplitude of 120 μmand a pitch of 350 μm on the front surface 56 by the press process, andproviding fine unevenness on the back surface 53 by the sand blastingmethod. The light adjusting sheet 50 is used such that the back surface53 which has fine unevenness is in contact with the light guiding plate32. The numeral 33 represents a linear light source provided on the edgeside of the light guiding plate 32. The numeral 38 represents areflector cover. The reflector cover 38 is installed behind the lightsource 33 and helps the light emitted from the light source 33efficiently enter the light guiding plate 32.

Next, operation of the light adjusting sheet 50 used in a planarlighting device of the edge-light type shown in FIGS. 17 and 18 aredescribed below. When the light source 33 is turned on, the lightemitted from the light source 33 enters the light guiding plate 32directly or after being reflected by the reflector cover 38. The lightwhich has entered the light guiding plate 32 emanates, directly or afterbeing reflected by the reflector plate 34, from the front surface as astrongly directional light (the angle from the light guiding plate 32 isapproximately 20°), as shown in FIG. 2. The light emanating from thefront surface of the light guiding plate 32 enters the light adjustingsheet 50. Here, since the back surface of the light adjusting sheet 50through which the light enters has the fine unevenness, the light isdiffused. Next, the light which has entered the light adjusting sheet 50travels through the light adjusting sheet 50 and goes out of its frontsurface. When the light goes out of the front surface, the direction ofthe light is altered to the forward direction by the convex streaks andthe concave streaks on the front surface.

The light distribution characteristics of the light which has travelledthrough the light adjusting sheet 50 is analyzed and the result is avery good distribution, indicated by H in FIG. 19. Further, theirregular reflection layer provided on the back surface of the lightguiding plate 32 is invisible when looking into the sheet 50.Additionally, when a liquid crystal display element is placed in frontof the planar lighting device using the light adjusting sheet, there isno poor readability caused by the irregular reflection layer and thescreen is very good-looking.

Embodiment 6

Next, Embodiment 6, as shown in FIG. 20 is described below. A lightadjusting sheet 70 shown in FIG. 20 has a front surface 71 provided withconvex streaks and concave streaks forming a sine curved surface and aback surface 72 provided with unevenness of mat surface forming a sinecurved surface by the sand blasting method. The pitch of the streaks ofthe front surface 71 is 400 μm and the amplitude is 200 μm. Theunevenness on the back surface 72 has the same pitch of 400 μm as thatof the front surface 71, but the amplitude is 20 μm. The thickness ofthe light adjusting sheet 70 is 500 μm. The light adjusting sheet 70 isfabricated by extruding a methylmethacrylic resin into a sheet using anextruder and then forming the uneven pattern on the sheet using anembossing roll.

The light adjusting sheet 70 shown in FIG. 20 is installed on the sameplanar lighting device as the one in Embodiment 5, and the lightdistribution characteristics are analyzed. The light distributioncharacteristics are somewhat different from the light distributioncharacteristics indicated by H in FIG. 19, but the distribution is verygood. When a liquid crystal display element is placed in front of thisplanar lighting device, the front surface of the liquid crystal displayelement is prevented from being in tight contact with the front surface71 of the light adjusting sheet 70. Therefore, there is no interferencefringe of Newton ring caused by the tight contact. Further, the light isconverged in the forward direction by the unevenness on the back surface72 to produce an even better looking screen than the planar lightingdevice in Embodiment 5.

Embodiment 7

Next, Embodiment 7 shown in FIG. 21 is described below. In Embodiment 7shown in FIG. 21, unevenness with a triangular cross section is providedon the back surface 82 opposite to the front surface 81 provided withconvex streaks and concave streaks forming a sine curved surface in thelight adjusting sheet 80. The unevenness of the back surface 82 has thesame pitch of 350 μm as the pitch on the front surface 81, but theamplitude is 50 μm. Other than this, the structure is the same as thatof Embodiment 5.

The light adjusting sheet 80 of Embodiment 7 shown in FIG. 21 isinstalled on the planar lighting device used in Embodiment 5, and aliquid crystal display element is placed in front of this planarlighting device. Since the front surface of the light guiding plate 32is prevented from being in tight contact with the back surface of theunevenness with a triangular cross section of the light adjusting sheet80. Therefore, there is no interference fringe of Newton ring caused bythe tight contact, and the light is better directed in the forwarddirection by the unevenness on the back surface 82, the result is aneven better looking screen than the planar lighting device of Embodiment5.

Embodiment 8

Next, Embodiment 8 shown in FIG. 22 is described below. Embodiment 8shown in FIG. 22 is different from Eembodiment 5 in that the lightadjusting sheet 50 includes 3 weight units of titanium oxide powdermixed in 100 weight units of polycarbonate by which the sheet mainlyconsists of. Other than this, the structure is the same as that ofEembodiment 5.

The light adjusting sheet 50 of Embodiment 8 shown in FIG. 22 isinstalled on the planar lighting device used in Embodiment 5, and aliquid crystal display element is placed in front of this planarlighting device. Since the light is diffused by the titanium oxidepowder, the irregular reflection layer provided on the back surface ofthe light guiding plate 32 is invisible, and the screen is betterlooking than the planar lighting device of Embodiment 5.

Embodiment 9

Next, Embodiment 9 shown in FIG. 23 is described below. The lightadjusting sheet 50 of Embodiment 9 shown in FIG. 23 is different fromEmbodiment 5 in that the back surface 57 opposite to the front surface56 provided with convex streaks and concave streaks forming a sinecurved surface is a flat surface. Other than this, the structure is thesame as that of Embodiment 5.

The light adjusting sheet 50 of Embodiment 9 shown in FIG. 23 isinstalled on the planar lighting device shown in FIG. 1, and a liquidcrystal display element is placed in front of this planar lightingdevice. Thus a good-looking, brighter than conventional, screen isobtained.

Next, the following testing is carried out in order to compare theperformance of conventional light adjusting sheets and those of thisinvention.

Comparative Testing

The light adjusting sheet of Embodiment 5 shown in FIG. 17 and that ofEmbodiment 6 shown in FIG. 20 are used as samples. For control samples,random unevenness of a 150 μm depth is provided on a sheet formed byextruding polycarbonate using an extruder, and fine unevenness isprovided, by the sand blasting method, on the back surface (Controlsample 4). The thickness of this light adjusting sheet is 200 μm.

The three samples described above are installed on the planar lightingdevice shown in FIG. 18, and the forward luminance (cd/m²), the anglerange (deg) in which the luminance brighter than a half of the forwardluminance, and the light emanation appearance quality are measured. Theresults are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                  Embodiment                                                                              Embodiment  Control                                                 5         6           sample 4                                      ______________________________________                                        Forward luminance                                                                         420         440         350                                       (cd/m.sup.2)                                                                  Angle range (deg)                                                                         45°  40°  55°                                Light emanating                                                                           ◯                                                                             ◯                                                                             ◯                             appearance quality                                                            ______________________________________                                    

As one can see in Table 2, the sheets of this invention, i.e.Embodiments 5 and 6, have a high forward luminance, a suitably diffusedangle range and a good light emanation appearance quality. Therefore,they are excellent sheets. On the other hand, Control Sample 4 has alarge angle range and a small forward luminance. Therefore, one can seethat it does not emanate the light efficiently in the direction of theuser's sight, and that a liquid crystal display element placed in frontof it would produce a dim screen.

Next, embodiments of the light adjusting sheet in which a plurality ofsheets each having convex streaks and concave streaks approximatelyparallel to each other on one surface are stacked will be described.

First, two types of sheets (g) (corresponding to the conventional lightadjusting sheet) and (h) (corresponding to a light adjusting sheetaccording to the invention) with different streak configurations areformed.

Sheet (g)

In FIG. 24, a plurality of parallel streaks 91 with a lateral crosssection of a right angled isosceles triangle are formed on the lightemanating surface of a square sheet (the peak angle is a right angle).The total thickness of the sheet is 300 μm and the height of the streaks91 is 150 μm. The material of the sheet is polycarbonate with a meltindex of 4.0 (290° C., 1.9 kg). The streaks 91 are formed by the hotpress method. Such is the configuration of the sheet (g) with aplurality of streaks 91.

Sheet (h)

In FIG. 25, a plurality of convex streaks 92 and concave streaks 93parallel to each other are formed on the light emanating surface of asquare sheet. Each of the convex streaks 92 has a lateral cross sectionof a convex arc with a radius of curvature of 20 μm, and each of theconcave streaks 93 has a lateral cross section of a concave arc with aradius of curvature of 20 μm. The total thickness of the sheet is 250 μmand the height of the convex streaks 92 is 120 μm. The material of thesheet is polymethyl methacrylate. Such is the configuration of the sheet(h) with a plurality of the convex streaks 92 and concave streaks 93.

The following sets of a cathode ray tube 33, a light guiding plate 32, areflector plate 34 and a light diffusing sheet 35 are used.

Set (A)

Cathode ray tube: One cold cathode ray tube of 3.5 mm diameter and 150mm length.

Light guiding plate: 150 mm long, 200 mm wide, 3.0 mm thick. Thematerial is polymethyl methacrylate. Dot printing is applied on thebottom surface.

Reflector plate: 100 μm thick. The material is polycarbonate mixed with20 wt % titanium oxide.

Light diffusing sheet: The material is polycarbonate mixed with 10 wt %calcium carbonate.

Set (B)

Cathode ray tubes: Two hot cathode ray tubes of 5.0 mm diameter and 170mm length.

Light guiding plate: 170 mm long, 230 mm wide, 5.0 mm thick. Thematerial is polycarbonate. Dot printing is applied on the bottomsurface.

Reflector plate: 100 μm thick. The material is polyethyleneterephthalate foam.

Light diffusing sheet: The material is polycarbonate mixed with 10 wt %calcium carbonate.

One or a stacked unit of two or more sheets (h) (and the sheet (g)additionaly) described above is used to compose light adjusting sheets,and these sheets are combined with the two types of sets (A) and (B).Each light adjusting sheet is placed in such a way that the peak linesof the convex streaks and the concave streaks are at the required angleto the axis of the cathode ray tube to compose a planar lighting device.

For example, two sheets 60 are placed on the light guiding plate 32 asshown in FIG. 26, or placed between the light guiding plate 32 and thelight diffusing sheet 35 as shown in FIG. 27.

Control Examples

Planar lighting devices in which one or two sheets (g) described aboveis placed in such a way that the peak lines of the convex streaks andthe concave streaks are parallel to the axis of the cathode ray tube arealso constructed.

For each planar lighting device of Embodiments and the control examples,the forward luminance and the luminance ratio are measured and theappearance is observed. The results are shown in Table 3. In Table 3:

* Angle of the streaks: The angle of the streaks is defined to be 0°when the peak lines of the streaks of the light adjusting sheets areparallel to the axis of the cathode ray tube.

* Luminance ratio (1): The ratio of the luminance to the highestluminance of the luminance measurements in varicus observationdirections for one luminescent device.

* Luminance ratio (2): The ratio of the luminance to the forwardluminance of the planar lighting device when the peak lines of thestreaks of one or multiple light adjusting sheets are placed parallel tothe axis of the cathode ray tube (with other conditions unchanged).

The luminance measurements are conducted with the procedure describedearlier.

                                      TABLE 3                                     __________________________________________________________________________                Embodiments           Control examples                                        10     11     12      5      6                                    __________________________________________________________________________    Set of cathode ray tube.                                                                  A      B      B       A      A                                    etc. (A or B)                                                                 Sheet (number of sheets)                                                                  h + h (2)                                                                            g + h (2)                                                                            h + h + h (3)                                                                         g (1)  g + g (2)                            *Peak angle of convex                                                                     15     +45    +30     0      0.0                                  andconcave streaks (°)                                                                    -45    -20                                                                           -50                                                 Position of light                                                                         Between light                                                                        Between light                                                                        Light emana-                                                                          Light emana-                                                                         Light emana-                         diffusing sheet                                                                           guiding plate                                                                        guiding plate                                                                        ting end                                                                              ting end                                                                             ting end                                         and light ad-                                                                        and light ad-                                                          justing sheet                                                                        justing sheet                                              Forward luminance                                                                         550    730    725     370    410                                  (cd/m.sup.2)                      (Insufficient                                                                 luminance)                                  *Luminance ratio 1                                                                        99%    98%    95%     65%    72%                                  *Luminance ratio 2                                                                        135%   140%   125%    --     --                                   Appearance  ◯                                                                        ◯                                                                        ◯                                                                         ◯                                                                        X Interfe-                                                                    nce pattern                                                                   (Moire)                              __________________________________________________________________________

Although the invention has been described with respect to specificembodiment for complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodification and alternative constructions that may occur to one skilledin the art which fairly fall within the basic teaching herein set forth.

What is claimed is:
 1. A planar lighting device comprising:a lightadjusting sheet having a plurality of convex regions and concave regionson one surface; each convex region and concave region being arrangedalternately and approximately parallel to each other; a light guidingplate having a dot pattern on a back surface being placed in back ofsaid light adjusting sheet; a linear light source placed on at least oneside of said light guiding plate; a reflector plate being placed in backof said light guiding plate; wherein peak lines of said convex regionand bottom lines of said concave regions of said light adjusting sheetare arranged to make a predetermined angle with an axis of said linearlight source; each of said convex regions having a cross section of aconvex arc and each of said concave regions having a cross section of aconcave arc; and wherein said light adjusting sheet comprises aplurality of single sheets each having a plurality of convex regions andconcave regions arranged alternately and approximately parallel to eachother, stacked together in such a way that peak lines of said convexregions and bottom lines of said concave regions of one single sheetmake an angle of 5° or greater with peak lines of said convex regionsand bottom lines of said concave regions of at least one other singlesheet.
 2. A planar lighting device, according to claim 1:wherein peaklines of said convex regions and bottom lines of said concave regionsmake an angle in the range of 5° to 85° either in the clockwisedirection or in the counterclockwise direction with an axis of saidlinear light source.
 3. A planar lighting device, according to claim1:wherein at least two of said single sheets are stacked so that saidpeak lines and said bottom lines of one single sheet makes an angle of5° or greater with said peak lines and said bottom lines of the othersingle sheet; said peak lines and said bottom lines of said one singlesheet makes an angle in the range of 5° to 85° in the clockwisedirection with an axis of said linear light source; and said peak linesand said bottom lines of said the other single sheet makes an angle inthe range of 5° to 85° in the counterclockwise direction with an axis ofsaid linear light source.
 4. A liquid crystal display comprising:a lightadjusting sheet having a plurality of convex regions and concave regionson one surface, each of said convex regions having a cross section of aconvex arc and each of said concave regions having a cross section of aconcave arc, each convex region and each concave region being arrangedalternately and approximately parallel to each other; a light guidingplate having a dot pattern on a back surface placed in back of saidlight adjusting sheet; a linear light source placed on at least one sideof said light guiding plate; a reflector plate being placed in back ofsaid light guiding plate; a liquid crystal display element being forwardof said light guiding plate; wherein peak lines of said convex regionsand bottom lines of said concave regions of said light adjusting sheetbeing arranged to make a predetermined angle with an axis of said linearlight source; said light adjusting sheet comprising a plurality ofsingle sheets each having a plurality of convex regions and concaveregions arranged alternately and approximately parallel to each other onone surface, stacked together in such a way that peak lines of saidconvex regions and bottom lines of said concave regions of one singlesheet make an angle of 5° or greater with peak lines of said convexregions and bottom lines of said concave regions of at least one othersingle sheet.
 5. A liquid crystal display, according to claim 4:whereinpeak lines of said convex regions and bottom lines of said concaveregions make an angle in the range of 5° to 85° either in the clockwisedirection or in the counterclockwise direction with an axis of saidlinear light source.
 6. A liquid crystal display, according to claim4:wherein at least two of said single sheets are stacked so that saidpeak lines and said bottoms lines of one single sheet makes an angle of5° or greater with said peak lines and said bottom lines of the othersingle sheet; said peak lines and said bottom lines of said one singlesheet make an angle in the range of 5° to 85° in the clockwise directionwith an axis of said linear light source; and said peak lines and saidbottom lines of said the other single sheet make an angle in the rangeof 5° to 85° in the counterclockwise direction with an axis of saidlinear light source.
 7. A planar lighting device comprising:a lightadjusting sheet having a plurality of convex regions and concave regionson one surface, each convex region and each concave region beingarranged alternately and approximately parallel to each other; a lightguiding plate having a dot pattern on a back surface being placed backof said light adjusting sheet; a linear light source placed on at leastone side of said light guiding plate; and a reflector plate being placedback of said light guiding plate; wherein peak lines of said convexregions and bottom lines of said concave regions of said light adjustingsheet being arranged to make a predetermined angle with an axis of saidlinear light source; wherein said light adjusting sheet comprises aplurality of single sheets each having a plurality of convex regions andconcave regions arranged alternately and approximately parallel to eachother, stacked together in such a way that peak lines of said convexregions and bottom lines of said concave regions of one single sheetmake an angle of 5° or greater with peak lines of said convex regionsand bottom lines of said concave regions of at least one other singlesheet.
 8. A planar lighting device, according to claim 7, wherein atleast two of said single sheets are stacked so that said peak lines andsaid bottom lines of one single sheet makes an angle of 5° degrees orgreater with said peak lines and said bottom lines of the other singlesheet;said peak lines and said bottom lines of said one single sheetmakes an angle in the range of 5° to 85° in the clockwise direction withan axis of said linear light source; and said peak lines and said bottomlines of said other single sheet makes an angle in the range of 5° to85° in the counterclockwise direction with an axis of said linear lightsource.